a. Field of the Invention
This invention relates to electro-acoustic transducers, for example micro speakers for use in reproducing sound in microelectronic equipment such as mobile phones, tablets, digital music players, navigation systems, laptop computers and the like. In particular, the invention relates to a stiffening plate for the membrane of an electro-acoustic transducer and a method of manufacturing such a stiffening plate.
b. Background Art
Electro-acoustic transducers used in microelectronic equipment have the ever increasing requirements of improved acoustic performance and decreased size of said transducers. The two requirements are often in conflict.
In miniature loudspeaker applications, where a membrane is driven by a voice coil, a low resonance frequency of the membrane is desired for obtaining good sound reproduction across a wide frequency range. A low resonance frequency can be achieved with a thin membrane having a relatively low Young's modulus. However, speakers with such membranes may have a low first break-up frequency, that is, the frequency at which a membrane may bulge and stop moving as a rigid piston. At the break-up frequency, a peak occurs in the frequency response representing a decreased performance of the speaker.
A known method of adjusting the first break-up frequency of a membrane is to provide damping by affixing a stiffening plate on top of the membrane. The material used for the plate must provide stiffness in order to increase the first break-up frequency, but must also be light weight to maintain the sensitivity of the membrane and not impact the loudness of the speaker. Composite stiffening plates, typically made of a polymer foam layer bonded between two metal layers by an adhesive, are known to have the necessary stiffness and low weight to provide effective damping to a membrane.
However, a desire for a smaller transducer, and in particular for one having a lower profile, cannot be met with known stiffening plates. Currently known commercially available composite stiffening plate material has a minimum thickness of 120 μm, the majority of which is the polymer foam layer. For a typical miniature loudspeaker, this may be 10 times more than the thickness of the membrane. There is a need, therefore, for a membrane stiffening plate with sufficient stiffness to provide damping to a membrane, of low weight and thinner than current known materials.